The invention relates generally to plasma-arc torch systems and power supplies. In particular, the invention relates to a circuit and method for controlling the pilot arc in a plasma-arc torch.
Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward a workpiece. An example of a conventional gas plasma-arc torch is disclosed in U.S. Pat. No. 3,813,510, the entire disclosure of which is incorporated herein by reference.
In general, a pressurized gas to be ionized is supplied to the front end of the torch and flows past an electrode before exiting through an orifice in a torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. The arc so established is commonly referred to as a pilot arc. A typical pilot arc circuit may provide 5-50 amps, at 100-200 volts across the electrode to tip gap.
In order to use a plasma-arc torch with a workpiece, a main or cutting arc must normally be established between the electrode and the workpiece. As the torch head or front end is brought toward the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative is typically lower than the impedance of the torch tip to negative. During this xe2x80x9ctransferred arcxe2x80x9d operation, the workpiece serves as the anode.
Once the arc transfer is sensed, it is generally preferred to cease current flow between the electrode and the tip. One method of terminating current flow between the electrode and the tip is to open circuit the pilot arc current path. This may be accomplished by sensing the presence of current flowing in the workpiece and open circuiting a switch between the tip and ground (positive return). Commonly owned U.S. Pat. Nos. 5,170, 030, and 5,530,220, the entire disclosures of which are incorporated herein by reference, describe the arc transfer process in greater detail.
After arc transfer occurs, the output current is typically increased to a higher, cutting level. The power supply preferably is current controlled so that the cutting current is maintained at or near a constant current level. If, however, the transferred arc is stretched beyond the capacity of the power supply it can extinguish. The arc may stretch, for example, when cutting a discontinuous workpiece (e.g., a metal grate), when cutting near the end of a workpiece, or when the torch is moved away from the workpiece. Once the arc has been extinguished, the torch starting process must typically be repeated. As can be appreciated, restarting the torch is relatively inefficient. Therefore, it is generally preferable to cause the pilot arc to re-attach before the transferred arc extinguishes.
U.S. Pat. No. 5,620,617 discloses an arc control circuit for a plasma-arc torch. A comparator compares the output voltage of the power supply to a maximum voltage. When the output voltage exceeds the maximum voltage, the comparator sets a logic device. The logic devices generates a signal to close a switch and reconnect the nozzle to the power supply, thereby switching the arc from the workpiece to the nozzle.
U.S. Pat. No. 5,844,197 discloses an arc retract circuit for use in a plasma-arc torch. The system disclosed therein involves creating a first signal representing the actual current applied by the power supply to the power circuit driving the plasma torch, creating a second signal representative of a current level below the set current level for the cutting operation, and closing a power switch in the pilot arc circuit when the first signal is essentially equal to the second signal.
The arc control/retract circuits disclosed in these patents require a comparison to a predefined reference (either voltage or current) in order to properly operate. Accordingly, such circuits are inherently limited by the reference chosen and require additional circuitry to establish the predefined reference.
For these reasons, a plasma-arc torch system having an improved pilot re-attach circuit and method is desired. Such a system and method requires the creation of no additional current or voltage reference signal in order to accurately sense when to cause the pilot arc to re-attach. Further, such a system and method preferably uses an existing signal to determine when to re-attach the pilot arc. Finally, such a system and method preferably provides a reliable and repeatable method of re-attaching the pilot arc prior to the extinguishment of the transferred arc.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved circuit and method for accurately determining whether and when to re-attach a pilot arc when a transferred arc may no longer be sustained. Advantageously, this is accomplished by monitoring the output of an already existing error amplifier output associated with a standard regulated power supply. Moreover, the improvement can be accomplished without the need for creating additional reference signals for comparison purposes that are not already present in a regulated power supply.
Briefly described, a plasma-arc torch system for use in connection with a workpiece embodying aspects of the invention includes a power source providing a power output current. An electrode is positioned in a circuit path with the power source. The output current flows through the electrode. A tip is adjacent the electrode. A current reference circuit provides a current reference signal that has a parameter indicative of a desired output current. A current regulator circuit provides an error signal that has a parameter indicative of a difference between the output current and the desired output current. A rate of change sensor receives the error signal. The rate of change sensor detects a rate of change in the error signal and selectively provides a switch control signal that has a parameter indicative of the rate of change in the error signal. A switch circuit is responsive to the switch control signal. The switch circuit selectively electrically connects the tip into the circuit path.
Another embodiment of a plasma-arc torch system for use with workpiece in accordance with the invention includes a power source providing an output current. An electrode is positioned in a circuit path with the power source. The electrode has the output current flowing therethrough. A tip is adjacent the electrode. A current reference circuit provides a current reference signal having a parameter indicative of the output current. An error amplifier compares the current reference signal to the sensed current signal and generates an error signal having a parameter indicative of a difference between the current reference signal and the sensed current signal. A rate of change sensor is electrically connected to the error amplifier. The rate of change sensor receives the error signal and selectively provides a switch control signal in response to a rate of change in the error signal. A switch circuit is responsive to the switch control signal. The switch circuit selectively electrically connects the tip into the circuit path with the power source and the electrode.
Another embodiment of the invention includes a pilot re-attach circuit for use in a plasma-arc torch system. The torch system includes a power source that provides an output current. An electrode is positioned in a circuit path with the power source and has the output current flowing therethrough. A tip is adjacent the electrode. An error amplifier generates an error signal in response to a difference between a signal representative of a sensed operating current and a signal representative of a desired operating current. The pilot re-attach circuit includes a rate of change sensor that receives the error signal and that is responsive to a time rate of change of the error signal. The rate of change sensor provides a switch control signal having a first state when the time rate of change of the error signal is less than a threshold. The rate of change signal has a second state when the time rate of change of the error signal exceeds the threshold. A pilot switch is responsive to the switch control signal. The pilot switch is operable to electrically connect the tip into the circuit path with the power source and the electrode when the switch control signal is in the second state.
In still another embodiment, the invention includes a plasma-arc torch system for use in connection with a workpiece. A power source means provides an output current. An electrode is positioned in a circuit path with the power source means. The electrode receives the output current. A tip is adjacent the electrode. A current reference means provide a current reference signal having a parameter indicative of a desired value of the output current. A current sensing means provides an actual output current signal having a parameter indicative of the output current. A comparing means compares the current reference signal to the actual output current signal. The comparing means generates an error signal having a parameter indicative of the difference between the current reference signal and the actual output current signal. A detector means is electrically connected to the error amplifier. The detector means receives the error signal and selectively provides a switch control signal in response to a rate of change in the error signal. A switching means is responsive to the switch control signal for selectively electrically connecting the tip into the circuit path with the power source means and the electrode.
Another embodiment of the invention includes a method of operating a plasma-arc torch system that includes a power supply that supplies an output current, and a pilot switch that establishes a pilot arc mode of operation. An output current signal having a parameter representative of the output current is compared to a reference signal having a parameter representative of a difference between the output current and the desired output current. An error signal is generated having a parameter representative of a difference between the output current and the desired output current. A rate of change of the error signal is monitored. The pilot switch is operated when the rate of change of the error signal exceeds a rate of change threshold such that the pilot arc mode of operation is established.
In still another embodiment, the invention includes a method of reestablishing a pilot arc in a plasma-arc torch system before a transferred arc is extinguished. The plasma-arc torch system includes an electrical power source providing an output current. An electrode receives the output current. A tip is adjacent the electrode. A pilot switch selectively connects the tip in a circuit path with the electrode and the power source such that when the pilot switch is closed, a pilot arc is selectively established between the electrode and the tip. The method includes monitoring an output current signal having a parameter representative of the output current provided by the electrical power source. A current reference signal is generated that has a parameter representative of a desired output current. The output current signal is compared to the current reference signal. An error signal is generated that is representative of a difference between the output current and the desired output current. A rate of change in the error signal is detected. A switch control signal is generated in response to the detected rate of change in the error signal. The switch control signal is representative of whether the detected rate of change in the error signal is greater than or less than a rate of change threshold. A pilot switch is operated in response to the rate of change signal such that when the rate of change is greater than the rate of change threshold, the pilot switch closes and connects the tip into the circuit path with the electrode and the power source.
A further embodiment of the invention includes a plasma-arc torch system for use in connection with a workpiece. A power source has a main regulator selectively providing an output current. An electrode is positioned in a circuit path with the power source and receives the output current. A tip is selectively connected into the circuit path with the power source and the electrode. An output current sensor provides an output current signal having a parameter indicative of the output current. A work current sensor provides a work current signal having a parameter indicative of the presence of work current flowing through the workpiece. The main regulator is responsive to the work current signal and the output current signal for regulating the output current to a first level when the work current signal indicates the presence of work current, and regulating the output current to a second level when the work current signal indicates the presence of no work current. A pilot regulator is responsive to the output current signal. The pilot regulator connects the tip into the circuit path with the power source and the electrode when the output current is less than a pilot regulator threshold. The pilot regulator disconnects the tip from the circuit path with the power source and the electrode when the output current is greater than the pilot regulator threshold.
In yet a further embodiment, the invention includes a method of operating a plasma arc torch system. The plasma-arc torch system includes a power supply providing an output current. An electrode receives the output current. A tip is adjacent the electrode. A switch selectively connects the tip into a circuit path with the power supply and the electrode. The torch system has a pilot mode of operation in which the power supply regulates the output current to a pilot current level. The torch system has a transferred arc mode of operation in which the power supply regulates the output to a cutting current level. The cutting current level is greater than the pilot current level. The method includes sensing the output current. The sensed output current is compared to an intermediate current level between the pilot current level and the cutting current level. The switch is configured as a pilot regulator for regulating a portion of the output current flowing through the tip to an intermediate level between the pilot current level and the cutting current level. The pilot regulator receives an output current signal indicative of the output current and connects the tip into the circuit path with the power supply and the electrode when the output current signal indicates that the output current is less than the intermediate current threshold. The pilot regulator disconnects the tip from the circuit path with the power supply and the electrode when the output current signal indicates that the output current is greater than the pilot regulator threshold.
Alternatively, the invention may comprise various other methods and systems.
Other objects and features will be in part apparent and in part pointed out hereinafter.